Slenderness requirements for railing posts

[MIKE_311]

I'm designing some railing posts for a pedestrian bridge. The PM/client wants very slender intermediate curved plate posts like this:

https://www.google.com/url?sa=i&url=https://

http://www.pinterest.co.uk/pin/1157...ved=0CAIQjRxqFwoTCNDdnL_8vO4CFQAAAAAdAAAAABAQ

or this:

https://i.pinimg.com/originals/fd/00/0f/fd000f36dc01e38035ef8ffb6095c232.jpg

How are these designed? Even though there is very little compressive force they don't look to meet the compression member limiting slenderness ratio of KL/r < 120 or even 140. I'm getting ratios up in near 300 for a plate that's 1/2" thick. 1" thick and I still can't get there. I am assuming a K=1.0

The cables provide no bracing support that I can see, they just go right through.

Is there some other assumption being made?

 

[BridgeSmith]

I think you'll have to analyze the posts as a slender column/compression strut. The slenderness limits you listed are for bridge primary structural members. For secondary structural tension members (those not included in the structural model), the limit is KL/r < 200.

K = 1.0 looks right to me.

Since what you're doing is not part of the structural system of the bridge, I don't think those slenderness limits are applicable.



Rod Smith, P.E., The artist formerly known as HotRod10

 

[STrctPono]

I wouldn't think that the standard slenderness limit applies to this situation. I put my vote in for a K value of 2.0 in the weak axis. You can probably simplify this and run a buckling hand check assuming a uniform cross section. Heck, I would probably just model that line post out of plate elements in a FEA program and run the buckling analysis there since it has such a unique shape. As you stated, the compressive loads are real light so I can't imagine this is an issue but a buckling analysis will prove that it is not.

Since those are line posts, they aren't taking any load from the cables, however, if your bridge is on a curve, it is more complicated and even the line posts will experience a lateral reactionary load when they tension the cables at the terminal posts.

 

[dik]

Easy to design... problem is horizontal members invite kids to climb... razor wire on top? I'd just design them for whatever the prescribed loading is and not be concerned about how it got there... also design the cables for the same loading with proper end anchorage...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ACtrafficengr]

If this in the US, how much cable tension is needed to pass the 4" sphere test? I assume other jurisdictions have similar requirements.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[MIKE_311]

This has been a bit of challenge to nail down. From talking to manufactures of cable systems, it requires 150 to 200 lbs, per cable if the cables are spaced at 3". Post spacing also needs to be about 4' unless you use add stabilizers at the posts.

I'm using 300 lbs per cable in the post design.

 

[bridgebuster]

Take a look at these fences, somwhat similar to what your client is seeking. These are used on NYC owned bridges.

 

[dik]

Thanks BB...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bridgebuster]

You're welcome dik. This is from a current project under construction. It has the regular traffic railing in front of the fancy fence.

 

[dik]

looks attractive and no razerwire needed... again thanks BB. just thought... jumper resistant.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

Fences on bridges over railroads have to be a minimum 8' high and curved or slanted in, or 10' high if they're straight. It's to try to prevent people from throwing things onto the trains.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

Thanks, Rod...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bridgebuster]

In the past Amtrak used to require an 8' barrier over electrified rails. It was usually a 4' concrete with a 4' solid metal barrier on top. Now they require 6'-6" concrete barrier with a 3' fence. We had a project in NYC over Amtrak 2 years ago, the 6'-6" barrier caused the DOT some grief. This was an interstate bridge, which would use a crash tested 3'-6" barrier; the 6'-6" barrier wasn't crash tested. Between us and DOT we were able to prove that it would work; no crash test.

 

[dik]

We lose more Toyotas, that way...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik